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Published by Perpus Kota Semarang, 2018-10-09 00:31:11

[Committee_on_Air_Quality_Management_in_U._S.] Air quality management in the United States

[Committee_on_Air_Quality_Management_in_U._S.] Air quality management in the United States

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

AIR QUALITY MANAGEMENT

IN THE UNITED STATES

Committee on Air Quality Management in the United States
Board on Environmental Studies and Toxicology
Board on Atmospheric Sciences and Climate
Division on Earth and Life Studies

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001

NOTICE: The project that is the subject of this report was approved by the Govern-
ing Board of the National Research Council, whose members are drawn from the
councils of the National Academy of Sciences, the National Academy of Engineer-
ing, and the Institute of Medicine. The members of the committee responsible for
the report were chosen for their special competences and with regard for appropriate
balance.

This project was supported by Grant No. X-82822101-0 between the National
Academy of Sciences and the U.S. Environmental Protection Agency. Any opinions,
findings, conclusions, or recommendations expressed in this publication are those of
the author(s) and do not necessarily reflect the view of the organizations or agencies
that provided support for this project.

Library of Congress Cataloging-in-Publication Data

Air quality management in the United States / Committee on Air Quality
Management in the United States, Board on Environmental Studies and
Toxicology, Board on Atmospheric Sciences and Climate, Division on Earth and
Life Studies.

p. cm.
Includes bibliographical references and index.
ISBN 0-309-08932-8 (hardback)—ISBN 0-309-51142-9 (pdf)
1. Air quality management—United States. I. National Research Council (U.S.).
Committee on Air Quality Management in the United States.
TD883.2.A64325 2004
363.739′25′0973—dc22

2004014594

Additional copies of this report are available from:

The National Academies Press
500 Fifth Street, NW
Box 285
Washington, DC 20055

800-624-6242
202-334-3313 (in the Washington metropolitan area)
http://www.nap.edu

Copyright 2004 by the National Academy of Sciences. All rights reserved.

Printed in the United States of America

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

The National Academy of Sciences is a private, nonprofit, self-perpetuating society
of distinguished scholars engaged in scientific and engineering research, dedicated to
the furtherance of science and technology and to their use for the general welfare.
Upon the authority of the charter granted to it by the Congress in 1863, the Acad-
emy has a mandate that requires it to advise the federal government on scientific and
technical matters. Dr. Bruce M. Alberts is president of the National Academy of
Sciences.

The National Academy of Engineering was established in 1964, under the charter of
the National Academy of Sciences, as a parallel organization of outstanding engi-
neers. It is autonomous in its administration and in the selection of its members,
sharing with the National Academy of Sciences the responsibility for advising the
federal government. The National Academy of Engineering also sponsors engineer-
ing programs aimed at meeting national needs, encourages education and research,
and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president
of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sci-
ences to secure the services of eminent members of appropriate professions in the
examination of policy matters pertaining to the health of the public. The Institute
acts under the responsibility given to the National Academy of Sciences by its con-
gressional charter to be an adviser to the federal government and, upon its own
initiative, to identify issues of medical care, research, and education. Dr. Harvey V.
Fineberg is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences
in 1916 to associate the broad community of science and technology with the
Academy’s purposes of furthering knowledge and advising the federal government.
Functioning in accordance with general policies determined by the Academy, the
Council has become the principal operating agency of both the National Academy of
Sciences and the National Academy of Engineering in providing services to the gov-
ernment, the public, and the scientific and engineering communities. The Council is
administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M.
Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National
Research Council.

www.national-academies.org

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

COMMITTEE ON AIR QUALITY MANAGEMENT
IN THE UNITED STATES

WILLIAM CHAMEIDES (Chair), Georgia Institute of Technology, Atlanta
DANIEL GREENBAUM (Vice-Chair), Health Effects Institute, Boston, MA
CARMEN BENKOVITZ, Brookhaven National Laboratory, Upton, NY
EULA BINGHAM, University of Cincinnati, Cincinnati, OH
MICHAEL BRADLEY, M.J. Bradley & Associates, Concord, MA
RICHARD BURNETT, Health Canada, Ottawa, Ontario
DALLAS BURTRAW, Resources for the Future, Washington, DC
LAURENCE CARETTO, California State University, Northridge
COSTEL DENSON, University of Delaware, Newark
CHARLES DRISCOLL, Syracuse University, Syracuse, NY
JANE HALL, California State University, Fullerton
PHILIP HOPKE, Clarkson University, Potsdam, NY
ARNOLD HOWITT, Harvard University, Cambridge, MA
C.S. KIANG, Peking University, Beijing, China
BEVERLY LAW, Oregon State University, Corvallis
JAMES LENTS, University of California, Riverside
DENISE MAUZERALL, Princeton University, Princeton, NJ
THOMAS MCGARITY, University of Texas School of Law, Austin
JANA MILFORD, University of Colorado, Boulder
MICHAEL MORRIS, North Central Texas Council of Governments, Arlington
SPYROS PANDIS, Carnegie Mellon University, Pittsburgh, PA
P. BARRY RYAN, Emory University, Atlanta, GA
ADEL SAROFIM, University of Utah, Salt Lake City
SVERRE VEDAL, National Jewish Medical and Research Center, Denver, CO
LAUREN ZEISE, California Environmental Protection Agency, Oakland

Project Staff

RAYMOND A. WASSEL, Project Director
LAURIE S. GELLER, Senior Program Officer
K. JOHN HOLMES, Senior Program Officer
AMANDA C. STAUDT, Senior Program Officer
KARL E. GUSTAVSON, Program Officer
CHAD A. TOLMAN, Staff Officer
RUTH E. CROSSGROVE, Editor
RAMYA CHARI, Research Assistant
MIRSADA KARALIC-LONCAREVIC, Research Assistant
EMILY L. BRADY, Senior Program Assistant
DOMINIC A. BROSE, Program Assistant

Sponsor: U.S. Environmental Protection Agency

v

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

BOARD ON ENVIRONMENTAL STUDIES AND TOXICOLOGY

Members

JONATHAN M. SAMET (Chair), Johns Hopkins University, Baltimore, MD
DAVID ALLEN, University of Texas, Austin
THOMAS BURKE, Johns Hopkins University, Baltimore, MD
JUDITH C. CHOW, Desert Research Institute, Reno, NV
COSTEL D. DENSON, University of Delaware, Newark
E. DONALD ELLIOTT, Willkie, Farr & Gallagher, LLP, Washington, DC
CHRISTOPHER B. FIELD, Carnegie Institute of Washington, Stanford, CA
WILLIAM H. GLAZE, Oregon Health and Science University, Beaverton
SHERRI W. GOODMAN, Center for Naval Analyses, Alexandria, VA
DANIEL S. GREENBAUM, Health Effects Institute, Cambridge, MA
ROGENE HENDERSON, Lovelace Respiratory Research Institute, Albuquerque, NM
CAROL HENRY, American Chemistry Council, Arlington, VA
ROBERT HUGGETT, Michigan State University, East Lansing
BARRY L. JOHNSON Emory University, Atlanta, GA
JAMES H. JOHNSON, Howard University, Washington, DC
JUDITH L. MEYER, University of Georgia, Athens
PATRICK Y. O’BRIEN, ChevronTexaco Energy Technology Company,

Richmond, CA
DOROTHY E. PATTON, International Life Sciences Institute, Washington, DC
STEWARD T.A. PICKETT, Institute of Ecosystem Studies, Millbrook, NY
ARMISTEAD G. RUSSELL, Georgia Institute of Technology, Atlanta
LOUISE M. RYAN, Harvard University, Boston, MA
KIRK SMITH, University of California, Berkeley
LISA SPEER, Natural Resources Defense Council, New York, NY
G. DAVID TILMAN, University of Minnesota, St. Paul
CHRIS G. WHIPPLE, Environ Incorporated, Emeryville, CA
LAUREN A. ZEISE, California Environmental Protection Agency, Oakland

Senior Staff

JAMES J. REISA, Director
DAVID J. POLICANSKY, Scholar
RAYMOND A. WASSEL, Senior Program Officer for Environmental Sciences

and Engineering
KULBIR BAKSHI, Senior Program Officer for Toxicology
ROBERTA M. WEDGE, Senior Program Officer for Risk Analysis
K. JOHN HOLMES, Senior Program Officer
SUSAN N.J. MARTEL, Senior Program Officer
SUZANNE VAN DRUNICK, Senior Program Officer
EILEEN N. ABT, Senior Program Officer
ELLEN K. MANTUS, Senior Program Officer
RUTH E. CROSSGROVE, Senior Editor

vi

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE

Members

ERIC J. BARRON (Chair), Pennsylvania State University, University Park
RAYMOND J. BAN, The Weather Channel, Inc., Atlanta, GA
ROBERT C. BEARDSLEY, Woods Hole Oceanographic Institution, Woods

Hole, MA
ROSINA M. BIERBAUM, University of Michigan, Ann Arbor
RAFAEL L. BRAS, Massachusetts Institute of Technology, Cambridge
CASSANDRA G. FESEN, Dartmouth College, Hanover, NH
MARGARET A. LEMONE, National Center for Atmospheric Research,

Boulder, CO
MARIO J. MOLINA, Massachusetts Institute of Technology, Cambridge
WILLIAM J. RANDEL, National Center for Atmospheric Research,

Boulder, CO
RICHARD D. ROSEN, Atmospheric & Environmental Research, Inc.,

Lexington, MA
JOHN C. WYNGAARD, Pennsylvania State University, University Park

Ex Officio Members

EUGENE M. RASMUSSON, University of Maryland, College Park
ERIC F. WOOD, Princeton University, Princeton, NJ

Staff

CHRIS ELFRING, Director
JULIE DEMUTH, Program Officer
SHELDON DROBOT, Program Officer
AMANDA STAUDT, Senior Program Officer
ELIZABETH A. GALINIS, Senior Program Assistant
ROB GREENWAY, Senior Program Assistant
DIANE L. GUSTAFSON, Administrative Coordinator
ROBIN A. MORRIS, Financial Associate

vii

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

OTHER REPORTS OF THE BOARD ON
ENVIRONMENTAL STUDIES AND TOXICOLOGY

Endangered and Threatened Fishes in the Klamath River Basin: Causes of
Decline and Strategies for Recovery (2004)

Cumulative Environmental Effects of Alaska North Slope Oil and Gas
Development (2003)

Estimating the Public Health Benefits of Proposed Air Pollution
Regulations (2002)

Biosolids Applied to Land: Advancing Standards and Practices (2002)
Ecological Dynamics on Yellowstone’s Northern Range (2002)
The Airliner Cabin Environment and Health of Passengers and Crew

(2002)
Arsenic in Drinking Water: 2001 Update (2001)
Evaluating Vehicle Emissions Inspection and Maintenance Programs

(2001)
Compensating for Wetland Losses Under the Clean Water Act (2001)
A Risk-Management Strategy for PCB-Contaminated Sediments (2001)
Acute Exposure Guideline Levels for Selected Airborne Chemicals

(3 volumes, 2000–2003)
Toxicological Effects of Methylmercury (2000)
Strengthening Science at the U.S. Environmental Protection Agency (2000)
Scientific Frontiers in Developmental Toxicology and Risk Assessment

(2000)
Ecological Indicators for the Nation (2000)
Modeling Mobile-Source Emissions (2000)
Waste Incineration and Public Health (1999)
Hormonally Active Agents in the Environment (1999)
Research Priorities for Airborne Particulate Matter (4 volumes,

1998–2003)
Ozone-Forming Potential of Reformulated Gasoline (1999)
Arsenic in Drinking Water (1999)
The National Research Council’s Committee on Toxicology: The First

50 Years (1997)
Carcinogens and Anticarcinogens in the Human Diet (1996)
Upstream: Salmon and Society in the Pacific Northwest (1996)
Science and the Endangered Species Act (1995)
Wetlands: Characteristics and Boundaries (1995)
Biologic Markers (5 volumes, 1989–1995)
Review of EPA’s Environmental Monitoring and Assessment Program

(3 volumes, 1994–1995)
Science and Judgment in Risk Assessment (1994)

viii

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Pesticides in the Diets of Infants and Children (1993)
Dolphins and the Tuna Industry (1992)
Science and the National Parks (1992)
Human Exposure Assessment for Airborne Pollutants (1991)
Rethinking the Ozone Problem in Urban and Regional Air Pollution

(1991)
Decline of the Sea Turtles (1990)
Copies of these reports may be ordered from The National Academies Press

(800) 624-6242 or (202) 334-3313
www.nap.edu

ix

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Preface

Recognizing the central role that science and engineering plays in air
quality management and anticipating the next congressional reauthoriza-
tion of the Clean Air Act and its amendments, the United States Congress
directed the U.S. Environmental Protection Agency (EPA) to arrange for a
study by the National Academy of Sciences (1) to evaluate from a scientific
and technical perspective the effectiveness of the major air quality provi-
sions of the Clean Air Act and their implementation by federal, state, tribal,
and local government agencies; and (2) to develop scientific and technical
recommendations for strengthening the nation’s air quality management sys-
tem with respect to the way it identifies and incorporates important sources
of exposure to humans and ecosystems and integrates new understandings
of human and ecosystem risks. In response, the National Research Council
established the Committee on Air Quality Management in the United States,
which prepared this report. Biosketches of the committee members are pre-
sented in Appendix A.

In the course of preparing this report, the committee met in public ses-
sions in Washington, D.C.; Denver, Colorado; Los Angeles, California; and
Atlanta, Georgia, where local, state, and federal officials and representatives
from the private sector and nongovernmental organizations, including regu-
lated industries and advocacy groups, were invited to meet with the commit-
tee and present their views on air quality management. Interested members
of the public at large were also given an opportunity to speak on these
occasions. The committee received oral and written presentations from the
following individuals:

xi

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Air Quality Management in the United States
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xii PREFACE

Daniel Albritton, National Oceanic and Atmospheric Administration
Aeronomy Laboratory; William Becker, Association of Local Air Pollution
Control Officials; Robert Brenner, EPA; Cynthia Burbank, Federal Highway
Administration; Tim Carmichael, Coalition for Clean Air; Michael Chang
and Rodney Weber, Georgia Institute of Technology; Patrick Cummins,
Western Governors’ Association; Gregory Dana, Alliance of Automobile
Manufacturers; Frank Danchetz, Georgia Department of Transportation;
Joan Denton, California Office of Environmental Health Hazard Assess-
ment; Howard Feldman, American Petroleum Institute; John Froines, Uni-
versity of California at Los Angeles; Mike Kenny, California Air Resources
Board; Eric Fujita, Desert Research Institute; Norma Glover and Barry
Wallerstein, South Coast Air Quality Management District; Charles
Goodman, Southern Company; Richard Jackson, Centers for Disease Con-
trol and Prevention; Kip Lipper, California State Senator Sher’s office;
Patricia Mariella, Gila River Indian Community; Barry McNutt, U.S. De-
partment of Energy; Christopher Miller, Environment and Public Works
Committee, U.S. Senate; Frank O’Donnell, Clean Air Trust; Harold Reheis,
Georgia Department of Natural Resources; Catherine Ross, Georgia Re-
gional Transportation Authority; Chet Tisdale, King and Spaulding; Paige
Tolbert, Emory University; Cindy Tuck, California Council for Environ-
mental and Economic Balance; Andrew Wheeler, Clean Air, Wetlands, Pri-
vate Property, and Nuclear Safety Subcommittee, U.S. Senate; and Robert
Yuhnke, Robert Yuhnke and Associates.

In addition to the information from those presentations, the committee
made use of the peer-reviewed scientific literature, government agency re-
ports, and unpublished databases, as well as related statistics and data di-
rectly obtained from EPA.

This report consists of seven chapters. The first chapter provides an
overview of the committee’s charge, the issues related to this charge, and the
approach the committee took in completing its task. Chapters 2–6 review
the current air quality management system in the United States and assess
how well this system is operating. Chapter 7 looks to the future; it identifies
the major air quality challenges the nation is likely to face in the coming
decade and advances a set of five interrelated recommendations for enhanc-
ing the nation’s air quality management system to meet these challenges.
The Executive Summary provides a brief overview of the committee’s find-
ings and recommendations. The more-detailed Summary is presented im-
mediately after the Executive Summary. Readers who are well versed in the
current operation of air quality management in the United States or who do
not need to become well versed may wish to move directly from the
Executive Summary or Summary to Chapter 7. The recommendations in

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

PREFACE xiii

Chapter 7 relate to Chapters 2–6, where the detailed background informa-
tion and justification for the recommendations are provided.

We wish to thank James Mahoney for his valuable service as a member
of the committee during the early stages of this study. He resigned appropri-
ately from the committee upon becoming assistant secretary of commerce
for oceans and atmosphere and deputy administrator of the National Oce-
anic and Atmospheric Administration. The committee’s work was assisted
by staff of the NRC’s Board on Environmental Studies and Toxicology
(BEST) and its Board on Atmospheric Science and Climate. We wish to
thank Raymond Wassel, project director, and James Reisa, director of BEST.
Scientific and technical information was provided by Laurie Geller, K. John
Holmes, Karl Gustavson, Amanda Staudt, Chad Tolman, Jhumoor Biswas,
Ramya Chari, Mirsada Karalic-Loncarevic, and Rachel Hoffman. Craig
Hicks assisted with science writing. Invaluable logistical support was pro-
vided by Emily Brady and Dominic Brose. The report was ably edited by
Ruth Crossgrove.

William L. Chameides, Chair
Daniel S. Greenbaum, Vice Chair

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Acknowledgment of Review Participants

This report has been reviewed in draft form by individuals chosen for
their diverse perspectives and technical expertise, in accordance with proce-
dures approved by NRC’s Report Review Committee. The purpose of this
independent review is to provide candid and critical comments that will
assist the institution in making its published report as sound as possible and
to ensure that the report meets institutional standards for objectivity, evi-
dence, and responsiveness to the study charge. The review comments and
draft manuscript remain confidential to protect the integrity of the delibera-
tive process. We wish to thank the following individuals for their review of
this report:

William Agnew, General Motors (retired); Thomas Burke, Johns
Hopkins University; Paul Crutzen, Max Planck Institute for Chemistry;
Gregory Dana, Alliance of Automobile Manufacturers; E. Donald Elliott,
Willkie, Farr & Gallagher, LLP; David Hawkins, Natural Resources De-
fense Council; Walter Heck, North Carolina State University; Timothy
Larson, University of Washington; Leonard Levin, Electric Power Research
Institute; Arthur Marin, Northeast States for Coordinated Air Use Manage-
ment; Michael Myer, Georgia Institute of Technology; Joseph Norbeck, Uni-
versity of California, Riverside; John Seitz, Sonnenschein, Nath &
Rosenthal, LLP; Thomas Tietenberg, Colby College; John Watson, Desert
Research Institute; Catherine Witherspoon, California Air Resources Board;
Terry Yosie, American Chemistry Council.

xv

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Air Quality Management in the United States
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xvi ACKNOWLEDGMENT OF REVIEW PARTICIPANTS
Although the reviewers listed above have provided many constructive

comments and suggestions, they were not asked to endorse the conclusions
or recommendations, nor did they see the final draft of the report before its
release. The review of this report was overseen by Robert Frosch, Harvard
University, and Edwin Clark II, Clean Sites. Appointed by the NRC, they
were responsible for making certain that an independent examination of
this report was carried out in accordance with institutional procedures and
that all review comments were carefully considered. Responsibility for the
final content of this report rests entirely with the authoring committee and
the institution.

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

Contents

Executive Summary 3

Summary 8

1 INTRODUCTION 23

Air Pollution Science, 24

Air Pollution Impacts, 28

Air Quality Management in the United States, 29

The Role of Science, 35

Estimating the Costs and Benefits of the Federally Mandated Air

Quality Management System, 37

The Future, 39

Charge to the Committee on Air Quality Management in the United

States, 41

Report Structure, 43

2 SETTING GOALS AND STANDARDS 45
Introduction, 45
Overview of Air Quality Standards, 46
The Standard-Setting Process, 47
Goals for Mitigating Visibility Degradation, 59
Standards for Mitigating Effects of Acid Rain, 59
The Scientific Basis for Setting Standards, 67
Summary, 86

xvii

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Air Quality Management in the United States
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xviii CONTENTS

3 DESIGNING AND IMPLEMENTING CONTROL STRATEGIES THROUGH 88
THE SIP PROCESS
Overview of SIP Process, 88
The Main Components of an Attainment-Demonstration SIP, 96
The Effectiveness of the SIP Process, 126
Summary, 131

4 IMPLEMENTING EMISSION CONTROLS ON MOBILE SOURCES 133

Introduction, 133

Controlling Emissions through Certification Standards on New

Vehicles and Motors, 136

Controlling In-Use Motor-Vehicle Emissions, 148

Behavioral and Societal Strategies to Reduce Mobile-Source

Emissions, 162

Critical Discussion of Mobile-Source Emission-Control Programs, 167

Summary, 172

5 IMPLEMENTING EMISSION CONTROLS ON STATIONARY SOURCES 174

Introduction, 174

Permits and Standards for New or Modified Major Stationary

Sources, 177

Other Technology-Based Standards Imposed on Major Facilities, 186

Evaluation of Traditional Control Programs for Major Stationary

Sources, 188

Compliance Assurance for Traditional Control Programs, 190

Cap-and-Trade Provisions for Major Stationary Sources, 196

Other Trading and Voluntary Stationary-Source Programs, 210

Area-Source Regulations, 212

Summary of Key Experiences and Challenges for Stationary-Source

Control, 214

6 MEASURING THE PROGRESS AND ASSESSING THE BENEFITS OF AQM 216

Introduction, 216

Monitoring Pollutant Emissions, 216

Monitoring Air Quality, 220

Assessing Health Benefits from Improved Air Quality, 241

Assessing Ecosystem Benefits from Improved Air Quality, 252

Assessing the Economic Benefits of Air Quality Improvements, 261

Summary, 265

7 TRANSFORMING THE NATION’S AQM SYSTEM TO MEET THE 268
CHALLENGES OF THE COMING DECADES
Introduction, 268
The Challenges Ahead, 270

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Air Quality Management in the United States
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CONTENTS xix

Principles for Enhancing the AQM System, 278 317
Recommendations for an Enhanced AQM System, 283 349
Conclusion, 313 355
363
References 365

Abbreviations 369

Appendix A COMMITTEE BIOSKETCHES

Appendix B STATEMENT OF TASK

Appendix C 188 HAZARDOUS AIR POLLUTANTS

Appendix D RECOMMENDATIONS FOR CONTINUOUS DEVELOPMENT
AND IMPLEMENTATION OF MEASUREMENTS TO DETERMINE
STATUS AND TRENDS IN ECOSYSTEM EXPOSURE AND
CONDITION

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Air Quality Management in the United States
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Figures and Tables

FIGURES
ES-1 Idealized schematic showing the iterative nature of air quality

management, 4
S-1 Comparison of growth areas and emission trends, 9
S-2 To meet the major challenges that will face air quality management

(AQM) in the coming decade, the committee identified a set of
overarching long-term objectives, 10
S-3 Plot of the estimated relative trends in emissions versus ambient
concentrations of various primary pollutants (PM10, NOx, SO2, Pb,
and CO), 14
1-1 Schematic of the factors influencing the pollutant mix in the
atmosphere and the resultant impacts of pollution, 25
1-2 National average emission categories for carbon monoxide (CO),
sulfur dioxide (SO2), nitric oxide and nitrogen dioxide (NOx),
volatile organic compounds (VOCs), particulate matter less than 10
micrometers in diameter (PM10), and particulate matter less than 2.5
micrometers in diameter (PM2.5) for 2001, 27
1-3 Idealized schematic showing three of the four sequential activities
carried out by the nation’s air quality management system, 34
1-4 Comparison of growth areas and emission trends, 38
1-5 Electricity generation by fuel in billion kilowatt hours, 1949–1999,
and projections for the Reference Case, 2000–2020, 40

xx

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FIGURES AND TABLES xxi

1-6 Counties in the continental United States where any NAAQS were
violated in 1999, 40

1-7 High cancer risk counties for urban air toxics in 1996 by
county, 41

1-8 Potential violations of the PM2.5 (1999–2000 data) and 8-hr O3
(1997–1999 data) NAAQS by county, 42

2-1 Flow diagram illustrating the process by which the EPA
administrator reviews and sets a new NAAQS, 50

2-2 Timeline illustrating historical sequence of the periodic NAAQS
reviews and final decisions carried out by EPA since the passage of
the 1970 CAA Amendments, 52

2-3 Foliar injury to cotton induced by chronic exposure to ozone, 54
2-4 The impact of haze on visibility, 60
2-5 Anthropogenic sources and natural sources contribute emissions

that result in the deposition of acidic compounds, 61
2-6 Trends in nationwide SO2 and NO2 emissions by year since

1940, 63
2-7 Schematic illustrating dose-response relationships between pollutant

exposure and (A) human health effects and (B) crop or vegetation
effects, 68
2-8 Exercising volunteer being exposed to ultrafine particles and
monitored for health response, 69
2-9 Evidence of health impact of ozone on human respiratory system
based on an experimental study involving human subjects, 70
2-10 Volunteer wearing a personal exposure monitor to measure actual
exposures to PM and gases during daily activities, 72
2-11 Four-chamber greenhouse-based exposure system constructed to
study effects of elevated CO2 on plants, 73
2-12 Studies in open-top field chambers have shown the response of
plants to ambient levels of O3, 74
2-13 Free air CO2 experiment (FACE) is used to elucidate forest
ecosystem responses to elevated CO2, 75
2-14 Concentration-response estimation from the reanalysis of the Pope/
American Cancer Society Study on cardiopulmonary disease
mortality (excluding Boise, Idaho), 77
2-15 Schematic diagram illustrating the source of human exposure to
indoor PM pollution, 84

3-1 Emission-inventory development, evaluation, and improvement, 101
3-2 Appendix J curve, 109
3-3 Empirical kinetic modeling approach (EKMA) diagram, 109

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xxii FIGURES AND TABLES

4-1 The evolution of California and federal tailpipe standards on
passenger car exhaust emissions since the 1960s, 138

4-2 Percentages of U.S. trucks within selected model years (MY) used
for various primary daily driving ranges: (1) up to 50 miles,
(2) 51 to 100 miles, (3) 101 to 200 miles, (4) 201 to 500 miles, and
(5) more than 500 miles, 154

4-3 Blood lead concentrations in the U.S. population from 1976 to
1999, 156

5-1 SO2 emissions from electric utilities in the United States from 1980
to 2001, 199

5-2 Regional SO2 emission from electric utilities, 206

6-1 Scatterplot of estimated trends in pollutant emissions derived
from emission inventories and changes in average pollutant
concentrations derived from air quality monitoring networks, 220

6-2 Locations of surface O3 monitoring sites and ozonesonde sites in
North America, 224

6-3 The PAMS network, 225
6-4 Locations of the National Atmospheric Deposition Program and

National Trends Network (NADP/NTN) monitoring sites in the
contiguous 48 United States, 229
6-5 National trend in annual benzene concentrations in metropolitan
areas, 1994–1999, 230
6-6 Trends in wet sulfate deposition in the United States using data from
the Clean Air Status and Trends Network (CASTNet) and the
National Atmospheric Deposition Program/National Trends
Network (NADP/NTN) (1989–1991 vs. 1997–1999), 231
6-7 (A) Total estimated U.S. lead emissions by major source category
from 1970 to 1994, 251

7-1 To meet the major challenges that will face air quality management
(AQM) in the coming decade, the committee identified a set of
overarching long-term objectives, 269

7-2 Contribution to the sulfate column burden for July 15, 1997, at
00UT (vertical integral of the concentration) from different source
regions showing intercontinental transport, 276

TABLES
1-1 Federal Air Quality Management Legislation, 30

2-1 NAAQS in Effect as of January 2003, 49

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FIGURES AND TABLES xxiii

3-1 Classification of Nonattainment Areas for O3 and CO Mandated in
the CAA Amendments of 1990, 92

3-2 Federal, State, and Local VOC Emission-Reduction Measures in
Four Illustrative SIPs, 118

3-3 Federal, Multistate, State, and Local NOx Emission-Reduction
Measures in Four Illustrative SIPs, 119

3-4 Classifications and Number of Nonattainment Areas in 1992
Remaining in Nonattainment As of February 6, 2003, 127

4-1 Types of Vehicles and Engines Regulated by AQM in the United
States, 134

4-2 Contribution of Nonroad Emissions to Mobile-Source Total and to
Manmade Total, 143

4-3A Average PM2.5 Emissions by Vehicle Model Years for Medium- and
Heavy-Duty Trucks, 152

4-3B Average NOx Emissions by Vehicle Model Years for Medium- and
Heavy-Duty Trucks, 153

4-4 Timeline of Significant Federal and State Regulations for Motor
Vehicle Fuels, 156

4-5 Part 1: California and Federal Reformulated Gasoline
Programs, 158
Part 2: Future Reformulated Gasoline Program, 159

5-1 Allowable Concentration Increments (micrograms per cubic meter)
for Prevention of Significant Deterioration (PSD), 182

5-2 NOx Emissions from Coal-Fired Boilers in 1999 by Vintage, 184
5-3 Open-Market and Other Noncapped Forms of Trading, 211

6-1 Summary of EPA’s Trends in Estimated Nationwide Pollutant
Emissions and Average Measured Concentrations, 218

6-2 Summary of Major U.S. Monitoring Networks, 222
6-3 Locations of Initial PM2.5 Supersites, 228
6-4 Ozone Monitoring Sites in the United States, 234

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AIR QUALITY MANAGEMENT

IN THE UNITED STATES

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Executive Summary1

The Clean Air Act (CAA) provides a legal framework for promoting
public health and public welfare2 by pursuing five major air quality goals
(see Box 1). For the first goal, the CAA authorizes the U.S. Environmental
Protection Agency (EPA) to set maximum allowable atmospheric concen-
trations of six major “criteria” pollutants by establishing National Ambient
Air Quality Standards (NAAQS). Individual states then develop state imple-
mentation plans (SIPs) that show how, with the assistance of national
control programs, they will meet these standards. Such efforts, as well as
those in pursuit of the other CAA goals, seek to regulate emissions from a
variety of stationary and mobile sources through the nation’s air quality
management (AQM) system (see Figure ES-1). Since passage of the CAA
Amendments of 1970, the nation has devoted significant efforts and re-
sources to AQM, and substantial progress has been made.

The Committee on Air Quality Management was formed by the Na-
tional Research Council to examine the role of science and technology in
the implementation of the CAA and to recommend ways in which the
scientific and technical foundations for AQM in the United States can be
enhanced. Over a 2-year period, the committee heard briefings from experts

1This Executive Summary provides a brief overview of the committee’s findings and recom-
mendations. The detailed Summary is presented after the Executive Summary.

2Within the framework of the CAA, “welfare” refers to the viability of agriculture and
ecosystems (such as forests and wildlands), the protection of materials (such as monuments
and buildings), and the maintenance of visibility.

3

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4 AIR QUALITY MANAGEMENT IN THE UNITED STATES

BOX 1 Goals of the Clean Air Act

• Mitigate potentially harmful ambient concentrations of six “criteria” pollut-
ants: carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone
(O3), particulate matter (PM), and lead (Pb).

• Limit sources of exposure to hazardous air pollutants (HAPs).
• Protect and improve visibility in wilderness areas and national parks.
• Reduce emissions of substances that cause acid deposition, specifically
sulfur dioxide and nitrogen oxides (NOx).
• Curb use of chemicals that have the potential to deplete the stratospheric
ozone layer.

3) Assessing Status and 1) Setting Standards
and Objectives
Measuring Progress
• Emissions standards
• Emissions trends • Ambient air quality standards
• Air quality trends • Reducing acid deposition
• Reducing regional pollution
• Health effects trends • Protecting visibility
• Ecosystem trends

• Institutional accountability Scientific and Technical

Foundation

Monitoring: Analysis:

· Emissions · Models (e.g., air

· Ambient air quality quality, emissions)

· Health and exposure · Economics

· Ecosystems · Health and ecological

· Meteorology risk assessment

Research: Development:
· Source control
· Public health and
ecosystems studies technology
· Monitoring technology
· Laboratory studies
(e.g., air chemistry,
toxicology)

2) Designing and Implementing
Control Strategies

• Source control technology requirements
• Emissions caps and trading
• Voluntary or incentive-based programs
• Energy efficiency
• Pollution prevention (e.g., product substitution and process alteration)
• Compliance assurance

FIGURE ES-1 Idealized schematic showing the iterative nature of air quality man-
agement. Bullets under each heading provide examples.

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EXECUTIVE SUMMARY 5

and stakeholders and examined the operation, successes, and limitations of
the many components of the nation’s AQM system.

PROGRESS

The committee concluded that implementation of the CAA has contrib-
uted to substantial decreases in emissions of several pollutants. Regulations
for light-duty vehicles, light-duty trucks, and fuel properties have greatly
reduced emissions per mile traveled. Programs for stationary sources, such
as power plants and large factories, have also achieved substantial reduc-
tions of pollutant emissions. However, most of the reductions have been
accomplished through regulations on new facilities, while many older, of-
ten higher-emitting facilities can be a substantial source of emissions. Emis-
sion “cap and trade” has also provided a mechanism for achieving emission
reductions at reduced costs. Air quality monitoring networks have con-
firmed that ambient pollutant concentrations, especially in urban areas,
have decreased over the past three decades, and monitoring has docu-
mented a reduction in sulfate deposition in the eastern United States. Eco-
nomic assessments of the overall costs and benefits of AQM in the United
States indicate, despite uncertainties, that implementation of the CAA has
had and will probably continue to have substantial net economic benefits.

CHALLENGES AHEAD

Despite the progress, the committee identified scientific and technical
limitations in the current AQM system that will hinder future progress,
especially as the nation attempts to meet the following key challenges in the
coming decade:

• Meeting new standards for ozone, particulate matter, and regional haze.
• Understanding and addressing the human health risks from expo-
sure to air toxics.
• Responding to the evidence that, for some pollutants, there may be
no identifiable threshold exposure below which harmful effects cease to
occur.
• Mitigating pollution effects that might disproportionately occur in
minority and low-income communities in densely populated urban areas.
• Enhancing understanding and protection of ecosystems affected by
air pollution.
• Understanding and addressing multistate and international trans-
port of pollutants.
• Adapting the AQM system to a changing (and most likely warmer)
climate.

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6 AIR QUALITY MANAGEMENT IN THE UNITED STATES

MEETING THE CHALLENGES:
THE COMMITTEE RECOMMENDATIONS

To meet these challenges and remedy current limitations, the committee
identified a set of overarching long-term objectives that should guide future
improvement of the AQM system. In the committee’s view, AQM should

• Strive to identify and assess more clearly the most significant expo-
sures, risks, and uncertainties.

• Strive to take an integrated multipollutant approach to controlling
emissions of pollutants posing the most significant risks.

• Strive to take an airshed3-based approach by assessing and control-
ling emissions of important pollutants arising from local, multistate, na-
tional, and international sources.

• Strive to emphasize results over process, create accountability for the
results, and dynamically adjust and correct the system as data on progress
are assessed.

Immediate attainment of these objectives is unrealistic. It would require
a level of scientific understanding that has yet to be developed, a commit-
ment of new resources that would be difficult to obtain in the short term,
and a rapid transformation of the AQM system that is undesirable in light
of the system’s past successes. The committee proposes, therefore, that the
AQM system be enhanced so that it steadily evolves toward meeting these
objectives. In that spirit, the committee makes five interrelated recommen-
dations to be implemented through specific actions:

1. Strengthen the scientific and technical capacity of the AQM system
to assess risk and track progress. Recommended actions include enhancing
assessments of air quality and health, ecosystem monitoring, emissions
tracking, exposure assessment (both outdoors and indoors), and other com-
ponents of the scientific and technical foundation of AQM.

2. Expand national and multistate performance-oriented control strat-
egies to support local, state, and tribal efforts. Recommended actions
include controlling currently unregulated and underregulated sources; ex-
panding use of performance-oriented, market-based (where appropriate)
multipollutant control strategies; and enhancing authority to identify and
address multistate and international air pollutant transport.

3. Transform the SIP process into a more dynamic and collaborative
performance-oriented, multipollutant air quality management plan (AQMP)

3Airshed is used here to denote the broader geographic extent of the emissions that contrib-
ute to the deleterious effects of a pollutant in a given location.

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EXECUTIVE SUMMARY 7

process. Recommended actions include enhancing the effectiveness and in-
novation of state and local air quality planning, while maintaining federal
oversight and retaining requirements for conformity with regional trans-
portation planning.

4. Develop an integrated program for criteria pollutants and hazardous
air pollutants (HAPs). Recommended actions include establishing a more
unified assessment of criteria and hazardous air pollutants, setting priorities
for those pollutants, establishing a more dynamic process for considering
new pollutants, and considering multiple pollutants in forming the scien-
tific basis for NAAQS.

5. Enhance protection of ecosystems and other aspects of public wel-
fare. Recommended actions include better tracking of ecosystem effects
and building an improved basis for implementing secondary or alternative
standards to protect ecosystems.

Implementation of these recommendations will still require substantial
resources, but they should not be overwhelming, especially when compared
with current expenditures for CAA compliance and costs resulting from
harmful effects of air pollution on human health and welfare. Implementing
these recommendations will also require a commitment by all parties to
adjust and change; it may also require new legislation from Congress. As the
transition occurs, however, it is imperative that ongoing programs to reduce
emissions continue so that progress toward cleaner air is maintained.

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Summary

Over the past three decades, the nation has devoted substantial efforts
and resources to protect and improve air quality through implementation
of the Clean Air Act (CAA). The U.S. Environmental Protection Agency
(EPA) estimates that the direct costs of this implementation have been as
high as $20–30 billion per year. There is little doubt that these expendi-
tures have helped reduce pollutant emissions despite the substantial in-
creases in activities that produce these emissions (see Figure S-1). Although
it is not possible to know what the exact concentrations of pollutant emis-
sions might be in the absence of the CAA, it is reasonable to conclude that
implementation of the act played an important role in lowering these emis-
sions. Cost-benefit analyses have generally concluded that the economic
value of the benefits to public health and welfare1 have equaled or exceeded
the costs of implementation.

Despite substantial progress in improving air quality, the problems
posed by pollutant emissions in the United States are by no means solved.
Future economic and population expansions and the concomitant increased
needs, for example, for electricity and transportation, will undoubtedly
increase the potential for emissions. Consequently, additional effort will
almost certainly be needed to maintain current air quality; even more effort
will be needed to make further improvements. The CAA prescribes a com-

1Within the framework of the CAA, “welfare” refers to the viability of agriculture and
ecosystems (such as forests and wildlands), the protection of materials (such as monuments
and buildings), and the maintenance of visibility.

8

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Air Quality Management in the United States 9
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SUMMARY

FIGURE S-1 Comparison of growth areas and emission trends. Note that the
trends in the graph (except for aggregate emissions) did not change substantially in
1995; only the scale of the graph changed. SOURCE: EPA 2002a.

plex set of responsibilities and relationships among federal, state, tribal,2
and local agencies for implementing the CAA. This is essentially the nation’s
air quality management (AQM) system.

CHARGE TO COMMITTEE
The Committee on Air Quality Management in the United States was
formed by the National Research Council in response to a congressional
request for an independent evaluation of the overall effectiveness of the
CAA and its implementation by federal, state, and local government agen-
cies. The committee was asked to develop scientific and technical recom-
mendations for strengthening the nation’s AQM system. In response to its
charge,3 the committee examined in detail the operation, successes, and
limitations of the many components of the nation’s AQM system and devel-
oped a set of unanimous findings and recommendations, as discussed below
and outlined in Figure S-2.

2Hereafter, “state” will be used to denote both state and tribal authorities.
3See Chapter 1 for a discussion of the committee’s approach to carrying out its charge.

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United StatesRECOMMENDATION 1. Strengthen Scientific and Technical
http://www.nap.edu/catalog/10728.htmlCapacity

10ACTIONS: Improve emissions tracking, enhance air pollution
monitoring, improve modeling, enhance exposure assessment,
Copyright © National Academy of Sciences. All rights reserved. improve health and welfare assessment, track implementation
costs, and invest in research and human and technical resources

The Challenges Ahead The Long-Term Objectives for RECOMMENDATION 2. Expand National and Multistate
AQM to Meet Future Challenges Control Strategies
· Meeting National Ambient Air
Quality Standards for ozone AQM should strive to: ACTIONS: Expand federal emission controls, emphasize
and particulate matter at 2.5 · Identify and assess most technology-neutral standards, use market-based approaches,
µm in diameter and reducing reduce existing-sources emissions, and address mulitistate
regional haze significant exposures, risks, and regional transport
uncertainties
· Designing and implementing · Take an integrated multipollutant RECOMMENDATION 3. Transform the SIP Process
controls for hazardous air approach to mitigating most
pollutants significant risks ACTIONS: Replace state implementation plan with integrated
· Take an airshed-based approach multipollutant air quality management plan and reform process to
· Protecting human health and to controlling emissions focus on tracking results using periodic reviews, encouraging
welfare in the absence of a · Emphasize results over process, innovative strategies, and retaining conformity and federal
threshold exposure create accountability, and oversight
dynamically adjust
· Ensuring environmental justice RECOMMENDATION 4. Develop Integrated Program for
· Assessing and protecting To begin the evolution of AQM Criteria and Hazardous Air Pollutants
toward these objectives in the
ecosystem health United States, five interrelated ACTIONS: Set priorities for pollutants, institute dynamic review of
· Mitigating intercontinental and Recommendations and related pollutant classification, list potentially dangerous but unregulated
Actions are proposed pollutants, address multipollutants in standard-setting process, and
cross-border transport enhance residual assessment
· Maintaining AQM system

efficiency in the face of
changing climate

RECOMMENDATION 5. Enhance Protection of Ecosystems
and Public Welfare

ACTIONS: Conduct review of standards to protect public welfare,
develop ecosystem monitoring networks, establish acceptable
ecosystem exposure levels, promulgate secondary standards, and
track progress

FIGURE S-2 To meet the major challenges that will face air quality management (AQM) in the coming decade, the committee
identified a set of overarching long-term objectives. Because immediate attainment of these objectives is unrealistic, the committee
made five interrelated recommendations to be implemented through specific actions.

Air Quality Management in the United States
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SUMMARY 11

THE CURRENT AQM SYSTEM

Two landmark events in 1970 helped to establish the basic framework
for managing air quality in the United States: the enactment of the CAA
Amendments and the creation of EPA. The CAA and its subsequent amend-
ments (such as those in 1977 and 1990) endeavor to protect and promote
public health and public welfare by pursuing the following goals:

• Mitigate potentially harmful ambient concentrations of six so-called
criteria pollutants: carbon monoxide (CO), nitrogen dioxide (NO2), sulfur
dioxide (SO2), ozone (O3), particulate matter (PM), and lead (Pb).

• Limit the sources of exposure to hazardous air pollutants (HAPs),
also called “air toxics.”

• Protect and improve visibility in wilderness areas and national parks.
• Reduce emissions of substances that cause acid deposition, specifi-
cally sulfur dioxide and nitrogen oxides (NOx).
• Curb use of chemicals that have the potential to deplete strato-
spheric ozone.4

The nation’s AQM system operates through three broad kinds of ac-
tivities (Figure ES-1): (1) setting standards and objectives, (2) designing and
implementing control strategies, and (3) assessing status and measuring
progress. The committee’s detailed assessments of the strengths and limita-
tions of these activities are presented in Chapter 2 (Setting Standards and
Objectives), Chapter 3 (Implementation Planning), Chapter 4 (Mobile-
Source Controls), Chapter 5 (Stationary-Source Controls), and Chapter 6
(Measuring Progress). Overall, the committee found that the AQM system
has made substantial progress, especially in the following ways:

• Setting National Ambient Air Quality Standards (NAAQS) for crite-
ria pollutants, designing and implementing state implementation plans (SIPs)
to comply with the NAAQS, and implementing other CAA programs to
address hazardous air pollutants, acid rain, and other issues have all pro-
moted enhanced technologies for pollution control and have contributed to
substantial decreases in pollutant emissions.

• Air quality monitoring networks have documented decreases in am-
bient concentrations of the criteria pollutants, especially in urban areas,
and despite growth in power production and transportation uses. The
NAAQS for sulfur dioxide, nitrogen dioxide, and carbon monoxide have

4The NRC charged the committee only to address air quality in the troposphere (lower
atmosphere). The NRC has elsewhere provided treatment of issues related to stratospheric
ozone depletion and global climate change, see NRC (1998a, 2001a,b, 2003a) and NAE/
NRC (2003).

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12 AIR QUALITY MANAGEMENT IN THE UNITED STATES

been largely attained. Monitoring networks have also documented a reduc-
tion in sulfate deposition in the eastern United States.

• Economic assessments of the overall costs and benefits of AQM in
the United States conclude that, even recognizing the considerable uncer-
tainties, implementation of the CAA has had net economic benefits.

With regard to the three broad activities in AQM (Figure ES-1), the
committee found the following:

Standard Setting

• Standard setting, planning and control strategies for criteria pollut-
ants and hazardous air pollutants have largely focused on single pollutants
instead of potentially more protective and more cost-effective multipollutant
strategies. Integrated assessments that consider multiple pollutants (ozone, par-
ticulate matter, and hazardous air pollutants) and multiple effects (health,
ecosystem, visibility, and global climate change) in a single approach are needed.

• Current risk assessment and standard-setting programs do not ac-
count sufficiently for all the hazardous air pollutants that may pose a
significant risk to human health and ecosystems or for the complete range
of human exposures both outdoors and indoors.

• EPA’s current practice for setting secondary standards5 for most
criteria pollutants does not appear to be sufficiently protective of sensitive
crops and ecosystems.

Designing and Implementing Controls

• Although pollutant concentrations have decreased, the federal, re-
gional, and state emission-control programs implemented under the SIP
process have not resulted in NAAQS attainment for ozone and particulate
matter in many areas. In addition, the SIP process has become overly
bureaucratic, places too much emphasis on uncertain emission-based mod-
eling simulations of future air pollution episodes, and has become a barrier
to technological and programmatic innovation.

• Air quality models have often played a major role in designing air
pollution control strategies. Much effort has gone into the development
and improvement of these models; as a result, they are highly sophisticated.
Limitations remain, however, in large part due to a lack of data to ad-
equately evaluate their performance in specific applications for specific
locations and an inability to rigorously quantify their uncertainty.

5Secondary standards are intended to protect against adverse public welfare effects (such as
deleterious effects on ecosystems and materials).

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SUMMARY 13

• Progress has been made in recognizing and addressing multistate
transport of air pollution, especially for ozone and atmospheric haze and
their precursors, in some parts of the nation. However, transport issues
need to be identified and addressed more proactively and the scope broad-
ened to include international transport.

• For mobile sources, regulations for light-duty vehicles, light-duty
trucks, and fuel properties have greatly reduced emissions per mile traveled.
Gaps remain, however, in the ability to monitor, predict, and control
vehicular emissions, especially from nonroad vehicles, heavy-duty diesel
trucks, and malfunctioning automobiles.

• Emission reductions from stationary sources (for example, power
plants and large factories) have also been substantial. However, most of
the reductions have been accomplished through regulations on new facili-
ties, while many older higher-emitting facilities continue to be a substantial
source of emissions.

• In recent years, emissions cap and trade has provided an effective
mechanism for achieving stationary-source emission reductions at reduced
costs. However, cap-and-trade programs have been limited to relatively
few pollutants, and the process of revising caps and targets in response to
new technical and scientific knowledge has been cumbersome.

Assessing Status and Measuring Progress

• With the exception of continuous emissions monitoring at some
large stationary sources, the nation’s AQM system lacks a comprehensive
and quantitative program to confirm the emission reductions claimed to
have occurred as a result of AQM.

• The air quality network in the United States is a national resource
but is nevertheless inadequate to meet important objectives, especially that
of tracking regional patterns of pollutant concentrations, transport, and
trends (see Figure S-3).

• The AQM system has not developed a program to track health and
ecosystem exposures and effects and to document improvements in health
and ecosystem outcomes achieved from improvements in air quality. Eco-
system effects have not been reliably and consistently accounted for in cost-
benefit analyses.

THE CHALLENGES AHEAD

Although the nation’s AQM system has been effective in addressing
some of the most serious air quality problems, it has a number of limitations,
as outlined above. In addressing how those limitations can best be remedied,

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14 AIR QUALITY MANAGEMENT IN THE UNITED STATES

FIGURE S-3 Plot of the estimated relative trends in emissions versus ambient con-
centrations of various primary pollutants (PM10, NOx, SO2, Pb, and CO). Emis-
sion trends, which were derived from emission inventories, are shown along the x-
axis, and the trends in average concentrations, which were derived from air quality
monitoring networks, are shown along the y-axis. The squares are the relative
trends in emissions and ambient concentrations for the 20-year period spanning
1983–2002 (except for PM10 emissions, which are for the trend period 1985–
2002), and the circles are the relative trends for the 10-year period of 1993–2002.
If the emission inventory trends were accurate and the nation’s air quality monitor-
ing networks were able to accurately measure the average concentration of primary
pollutants in the air overlying the United States, all the points on the graph would
fall on the 1:1 (diagonal) line. However, the fact that most of the points on the
graph do not fall on the 1:1 line indicates that the emission inventory trends are
inaccurate and/or that the nation’s air quality network, which was initially de-
signed to monitor urban pollution and compliance with NAAQS, has not been able
to track trends in pollutant concentrations quantitatively across urban, suburban,
and rural settings. Despite such uncertainty, it is important to note that the down-
ward trend in ambient pollutant concentrations provides qualitative confirmation
that pollutant emissions have been decreasing. SOURCE: Data from EPA 2003.

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SUMMARY 15

it is important to consider the major air quality challenges facing the nation
in the coming decades. Seven major challenges are outlined below.

• New Standards. Additional reductions in pollutant emissions will
be required to meet the EPA 1997 standards for ozone and particulate
matter and the 1999 regulations for regional haze. Improvements to the
AQM system will be needed to best identify what emissions to reduce and
to monitor the progress toward meeting new standards.

• Toxic Air Pollutants. The human health risks from exposure to
toxic pollutants remain significant and poorly quantified. A greater re-
search effort that focuses on the sources, atmospheric distribution, and
effects of most toxic air pollutants will be needed to address health risks
and ensure adequate protection to the public.

• Health Effects at Low Pollutant Concentrations. There is increasing
evidence that there might not be an identifiable exposure concentration
(threshold) for some criteria pollutants below which human health effects
would cease to occur. A better understanding of the reducible (human-
induced) and irreducible components of pollution, as well as the health and
ecosystem impacts at low levels of exposure, is needed. Once improved
scientific understanding is developed, it might be necessary to reconsider
how to set standards to protect public health from pollutants for which
thresholds can not be identified.

• Environmental Justice. The CAA does not have any programs ex-
plicitly aimed at mitigating pollution effects that might be borne dispropor-
tionately by minority and low-income communities in densely populated
urban areas. Addressing this need will require enhancing the science base
for determining exposures of selected communities to air pollution and
incorporating environmental equity concepts in the earliest stages of air
quality planning. Native American tribes should be given help to develop
and implement AQM programs for reasons of environmental justice and
tribal self-determination.

• Protecting Ecosystem Health. Although mandated in the CAA, the
protection of ecosystems affected by air pollution has not received appro-
priate attention in the implementation of the act. A research and monitor-
ing program is needed that can quantify the effects of air pollution on the
structure and functions of ecosystems. That information can be used to
establish realistic and protective goals, standards, and implementation strat-
egies for ecosystem protection.

• Multistate, Cross-Border, and Intercontinental Transport. Evidence
is accumulating that shows that air quality in a specific area can be influ-
enced by pollutant transport across multistate regions, national boundaries,
and continents. To address multistate pollutant transport, the AQM sys-
tem must improve the techniques for tracking and documenting pollutant

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16 AIR QUALITY MANAGEMENT IN THE UNITED STATES

transport and develop more effective mechanisms for coordinating multi-
state regional air pollution control strategies. In addition, the nation should
continue to pursue collaborative projects and enter into agreements and
treaties with other nations to help minimize pollution transport to and from
the United States.

• AQM and Climate Change. The earth’s climate is warming. Al-
though uncertainties remain, the general consensus within the scientific
community is that this warming trend will continue or even accelerate in
the coming decades. The AQM system will need to ensure that pollution
reduction strategies remain effective as the climate changes, because some
forms of air pollution, such as ground-level ozone, might be exacerbated. In
addition, emissions that contribute to air pollution and climate change are
fostered by similar anthropogenic activities, that is, fossil fuel burning.
Multipollutant approaches that include reducing emissions contributing to
climate warming as well as air pollution may prove to be desirable.

RECOMMENDATIONS

To meet the challenges of the coming decades and remedy current
limitations, the committee identified a set of long-term, overarching objec-
tives to guide future improvement of the AQM system. In the committee’s
view, AQM should

• Strive to identify and assess more clearly the most significant expo-
sures, risks, and uncertainties.

• Strive to take an integrated multipollutant approach to controlling
emissions of pollutants posing the most significant risks.

• Strive to take an airshed6-based approach by assessing and control-
ling emissions of important pollutants arising from local, multistate, na-
tional, and international sources.

• Strive to emphasize results over process, create accountability for the
results, and dynamically adjust and correct the system as data on progress
are assessed.

Immediate attainment of these objectives is unrealistic. It would require
a level of scientific understanding that has yet to be developed, a commit-
ment of new resources that would be difficult to obtain in the short term,
and a rapid transformation of the AQM system that is uncalled for in light
of the system’s past successes. The committee proposes, therefore, that the
AQM system be enhanced so that it steadily evolves towards meeting these

6Airshed is used here to denote the broader geographic extent of the emissions that contrib-
ute to the deleterious effects of a pollutant in a given location.

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Air Quality Management in the United States
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SUMMARY 17

objectives. In that spirit, the committee makes five interrelated recommen-
dations, to be implemented in concert through some 30 specific actions
described in this report.

Recommendation One

Strengthen scientific and technical capacity to assess risk and track
progress.

Improving the nation’s AQM system will depend heavily on reassessing
and investing in relevant scientific and technical capacity to help evolve the
AQM system to one that can focus on risk in priority setting and on per-
formance in measuring progress. Without the enhancement of the nation’s
scientific and technical capacity, implementation of the other four recom-
mendations will be more difficult. The most critical actions are

• Improve emissions tracking, including new emissions monitoring tech-
niques and regularly updated and field-evaluated inventories.

• Enhance air pollution monitoring, including new monitoring meth-
ods, expanded geographic coverage, improved trend analysis, and enhanced
data accessibility.

• Improve modeling, including enhanced emission and air measure-
ment programs to provide data for model inputs and model evaluation and
continued development of shared modeling resources.

• Enhance exposure assessment, including improved techniques for mea-
suring personal and ecosystem exposure and designing strategies to control
the most significant sources of ambient, hot-spot,7 and indoor exposures.

• Develop and implement a system to assess and monitor human health
and welfare effects through the identification of indicators capable of char-
acterizing and tracking the effects of criteria pollutants and hazardous air
pollutants and the benefits of pollution control measures and their sus-
tained use in assessments, such as the 2003 EPA Draft Report on the
Environment.

• Continue to track implementation costs by supporting the Pollution
Abatement Cost and Expenditure (PACE) survey and conducting detailed

7Hot spots are locales where pollutant concentrations are substantially higher than concen-
trations indicated by ambient outdoor monitors located in adjacent or surrounding areas. Hot
spots can occur in indoor areas (for example, public buildings, schools, homes, and factories),
inside vehicles (for example, cars, buses, and airplanes), and outdoor microenvironments (for
example, a busy intersection, a tunnel, a depressed roadway canyon, toll plazas, truck termi-
nals, airport aprons, or nearby one or many stationary sources). The pollutant concentrations
within hot spots can vary over time depending on various factors including the emission rates,
activity levels of contributing sources, and meteorological conditions.

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Air Quality Management in the United States
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18 AIR QUALITY MANAGEMENT IN THE UNITED STATES

and periodic examinations of actual costs incurred in a subset of past
regulatory programs, including comparisons of actual costs to the costs
predicted by various parties prior to adoption of regulations.

• Invest in research to facilitate multipollutant approaches that targets
the most significant risks, including enhanced research into the full range of
ambient, hot-spot, and indoor exposures and their potential risks.

• Invest in human and technical resources through programs and in-
centives to attract and train a diverse corps of scientists and engineers
contributing to AQM and the development of an environmental extension
service.

Recommendation Two

Expand national and multistate performance-oriented control strategies
to support local, state, and tribal efforts.

The role of EPA in establishing and implementing national and multi-
state emission-control measures should be expanded so that states can
focus their efforts on local emission concerns. The most critical actions
are

• Expand federal emission-control measures especially for nonroad
mobile sources (for example, aircraft, ships, locomotives, and construction
equipment), area sources (relatively small dispersion), and building and
consumer products. Development of these measures should actively involve
states, local agencies, and stakeholders and allow for continued control-
measure innovation at the state and local level.

• Emphasize technology-neutral standards for emission control. When-
ever practical, control measures should cap the total emissions from a given
source or group of sources, as opposed to limiting the rate of emissions
per unit of resource input or product output. In cases where a cap is not
practical, standards should be set that promote improved technologies
rather than being tied to a single technology and that are stringent enough
to offset projected emission increases caused by future growth in economic
activity.

• Use market-based approaches whenever practical and effective
through the expanded use of approaches, such as the acid rain SO2 emis-
sions cap-and-trade program, that have the potential to be highly effective
and realize substantial cost savings. Such programs must incorporate con-
tinuous emissions monitoring to ensure that emission goals are met and be
designed to identify and minimize geographic and temporal disparities in
results. Expansion to new industrial sectors will require enhanced continu-
ous emission-monitoring systems, technologies to ensure that required re-

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
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SUMMARY 19

ductions are achieved, and ambient monitoring to establish that the pro-
gram does not inadvertently result in geographic and temporal disparities in
results.

• Reduce emissions from existing facilities and vehicles, to the extent
practical by promulgating standards for sources regardless of their age,
status, or fuel. Older stationary sources and mobile nonroad sources are of
particular concern.

• Address multistate transport problems by providing EPA with greater
statutory responsibility to assess multistate air quality issues on an ongoing
basis and the regulatory authority to deal with them in a regional context.
Constitutionally, interstate environmental rules and regulations must be based
on federal authority, but EPA has not been given sufficient tools under the
CAA to address the multistate aspects of most air quality problems.

Recommendation Three

Transform the SIP process.

Implementation planning at the state and local levels should be changed
to place greater emphasis on performance and results and to facilitate
development of multipollutant strategies. Critical actions include

• Transform the SIP into an AQM plan. Each state should be required
to prepare an air quality management plan (AQMP) that integrates the
relevant air quality measures and activities into a single, internally consis-
tent plan. An evolution of the SIP process to an AQMP approach should
involve the following:

—Given the similarity of sources, precursors, and control strate-
gies, the AQMP should encompass all criteria pollutants in an inte-
grated multipollutant plan.

—EPA should identify key hazardous air pollutants that have di-
verse sources or substantial public health impacts. These pollutants
should be included in an integrated multipollutant control strategy and
addressed in each state’s AQMP.

—The scope of the AQMP should explicitly identify and propose
control strategies for air pollution hot spots and situations where disad-
vantaged groups may be disproportionately exposed and should pro-
vide incentives to implement the strategies.

—Given the current statutory requirements and rules associated
with the SIP, it might be necessary to implement this recommendation
in stages and provide incentives to facilitate the transition to an AQMP
approach.

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20 AIR QUALITY MANAGEMENT IN THE UNITED STATES

• Reform the planning and implementation process by

—Encouraging regulatory agencies to concentrate their resources
on tracking and assessing the performance of the strategies that have
been implemented rather than on preparing detailed documents to jus-
tify the effectiveness of strategies in advance of their implementation.

—Carrying out a formal and periodic process of review and re-
analysis of the AQMP to identify and implement revisions to the plan
when progress toward attainment of standards falls below expectations
or when conditions change sufficiently to invalidate the underlying
assumptions of the plan. Given the large contributions of federal and
multistate measures to the success of any plan, it is essential that this
review process be collaborative and include all relevant federal and
state agencies.

—Encouraging the development and testing of innovative strategies
and technologies by not requiring predetermined and agreed-upon ben-
efits for every strategy but periodically evaluating their effectiveness.

—Retaining the federal requirement for conformity between air
quality planning and transportation planning. Conformity could be
improved by mandating greater consistency between the data, models,
and time frames used in air quality and transportation plans.

—Continuing to require that states implement agreed upon strategies,
ensure private-sector compliance, and are held accountable for failure to
meet the AQMP commitments through federally mandated sanctions.

Recommendation Four

Develop an integrated program for criteria pollutants and hazardous air
pollutants.

The time has come for the nation’s AQM system to begin the transi-
tion toward an integrated, multipollutant approach that targets the most
significant exposures and risks. The critical actions include

• Develop a system to set priorities for hazardous air pollutants by
expanding the approach embodied in EPA’s urban air toxics program. As
proposed in Recommendation Three, a few hazardous air pollutants, be-
cause of their diverse sources, ubiquitous presence in the atmosphere, or
exceptionally high risk to human health and welfare, might warrant treat-
ment similar to criteria pollutants and be included in AQMPs.

• Institute a dynamic review of pollutant classification, and reclassify
and revise priorities for criteria pollutants and hazardous air pollutants
accordingly.

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
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SUMMARY 21

• List potentially dangerous but unregulated air pollutants for regula-
tory attention. Determine whether there are sufficient data on adverse
impacts, chemical structure, and potential for population exposure and
identify whether some level of regulatory response would be prudent and
appropriate.

• Address multiple pollutants in the NAAQS review and standard-
setting process by beginning to review and develop NAAQS for related
pollutants simultaneously.

• Enhance assessment of residual risk by performing an increased num-
ber of assessments in the years to come and by attempting to include in the
assessments other major sources of the same chemicals.

Recommendation Five

Enhance protection of ecosystems and other aspects of public welfare.

Many of the programs and actions undertaken in response to the CAA
have focused almost entirely on the protection of human health. Further
efforts are needed to protect ecosystems and other aspects of public welfare.
The critical actions include

• Completion of a comprehensive review of standards to protect pub-
lic welfare.

• Develop and implement networks for comprehensive ecosystem
monitoring to quantify the exposure of natural and managed resources to
air pollution and the effects of air pollutants on ecosystems.

• Establish acceptable exposure levels for natural and managed eco-
systems by evaluating data on the effects of air pollutants on ecosystems at
least every 10 years.

• Promulgate secondary standards where needed that take the appro-
priate form. For example, in some cases a standard based on the amount of
a pollutant that is deposited on the earth’s surface over a particular area
may be more appropriate than a standard based on the atmospheric con-
centration of that pollutant. Allow for consideration of regionally distinct
standards.

• Track progress toward attainment of secondary standards by using
the aforementioned monitoring of ecosystem exposure and response.

MOVING FORWARD

Because the nation’s AQM system has been effective in many aspects
over the past three decades, much of the system is good and warrants
retaining. Thus, the recommendations proposed here are intended to evolve

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
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22 AIR QUALITY MANAGEMENT IN THE UNITED STATES

the AQM system incrementally rather than to transform it radically. The
recommendations are also not intended to deter current ongoing AQM
activities aimed at improving air quality. Indeed, even as these recommen-
dations are implemented, there can be little doubt that important decisions
to safeguard public health and welfare should continue to be made, often in
the face of scientific uncertainty. Moreover, new opportunities and ap-
proaches for managing air quality will appear. These include addressing
pollution problems in multiple environmental media, such as air and water,
taking advantage of new technologies, and undertaking pollution preven-
tion activities rather than controlling air pollutants after they have been
produced.

Implementation of the recommendations will require the development
of a detailed plan and schedule of steps. The committee urges EPA to
convene an implementation task force from the key parties to prepare a
plan of action and an analysis of legislative actions, if any are needed.

Implementation of the recommendations will also require additional
resources. Although these resources are not insignificant, they should not
be overwhelming. For example, consider the costs associated with air qual-
ity research and monitoring. Even a doubling of the approximate $200
million in EPA funds currently dedicated to air quality monitoring and
research would represent about 1% of annual expenditures nationwide for
complying with the CAA. Such resources are even smaller when compared
with the costs imposed by the deleterious effects of air pollution on human
health and welfare.

Implementation of the recommendations will require a commitment by
all parties to stages of implementation over several years. As that transition
occurs, it is important that action on individual programs to reduce emis-
sions continues to maintain progress toward cleaner air.

The full complement of scientific and engineering disciplines will need
to be prepared to take up the substantial challenges embodied in these
recommendations. Given the opportunity, the committee believes that the
scientific and engineering communities can provide the human resources
and technologies needed to underpin an enhanced AQM system and to
achieve clean air in the most expeditious and effective way possible.

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States
http://www.nap.edu/catalog/10728.html

1

Introduction

The goal of protecting and enhancing air quality to protect and pro-
mote human health and public welfare1 has been consistently set forward in
the United States during the latter part of the twentieth century. To accom-
plish this goal, numerous regulations and standards, a broad suite of man-
agement tools, and several monitoring networks to track progress have
been established. All of these components depend on robust and up-to-date
scientific and technical input, which includes an understanding of relation-
ships between air pollutant levels and impacts on human health, ecosys-
tems, atmospheric visibility, and materials. The National Research Council
Committee on Air Quality Management in the United States was asked to
evaluate the effectiveness of the nation’s air quality management (AQM)
system and the extent to which it is informed by the most advanced science
and technology. This chapter begins with a brief summary of the current
scientific and technical understanding of air pollution and its impacts, as
well as an overview of the AQM system in the United States and the federal
legislation that has motivated and driven much of its development in the
latter part of the twentieth century. This overview discussion is intended to
provide an introduction to aspects of the AQM system that are described in
more detail and critiqued in later chapters of this report. The report is not
intended to provide a comprehensive description of AQM activities in the

1Within the framework of the Clean Air Act, “welfare” refers to the viability of agriculture
and ecosystems (such as forests and wildlands), the protection of materials (such as monu-
ments and buildings), and the maintenance of visibility.

23

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24 AIR QUALITY MANAGEMENT IN THE UNITED STATES

United States; pertinent references are provided for those who desire more
background information on general concepts of scientific and technical
understanding of air pollution and its impacts.

AIR POLLUTION SCIENCE

The atmosphere is composed of a mixture of gases and particles. An air
pollutant is generally defined as any substance in air that, in high enough
concentrations, harms humans, ecosystems (other animals and vegetation),
or materials (such as buildings and monuments) and reduces visibility. In
this report, the committee uses the term air pollutant to denote the subset of
harmful atmospheric substances that are present, at least in part, because of
human activities rather than natural production and whose principal del-
eterious effects occur as a result of exposure at ground level. Greenhouse
gases, as well as pollutants that cause depletion of ozone (O3) in the strato-
sphere, the layer of atmosphere extending from about 10 to 16 kilometers
(km) up to 50 km altitude, are addressed only in the context of managing
ground-level air quality.2

The science of air pollution is primarily concerned with quantitatively
understanding the so-called “source-receptor relationships” that link specific
pollutant emissions to the pollutant concentrations and deposition observed
in the environment as a function of space and time. This quantitative under-
standing is developed through extensive field and laboratory measurements
and analysis and is then tested and documented in air quality models that use
mathematical and numerical techniques to simulate the physical and chemi-
cal processes that affect air pollutants as they disperse and react in the atmo-
sphere. As illustrated in Figure 1-1, the pollutants at a particular time and
place depend on the proximity to sources that emit pollutants or their precur-
sors; the chemical reactions that pollutants or their precursors undergo once
in the atmosphere; and the impact of mixing, dilution, transport, and re-
moval or deposition processes (Seinfeld and Pandis 1998). The areas of air
quality science and air quality management are closely coupled, because the
tools developed by scientists and engineers to carry out the tasks described
above are also widely used by the agencies tasked with controlling air pollu-
tion. For example, the instrumentation used by scientists in field experiments
is also used by regulatory agencies to monitor air pollution exposures, trends,
and compliance. Similarly, the models developed by scientists to simulate
and better understand air pollution are used in AQM to help design effective
strategies for air pollution mitigation.

2The NRC charged the committee to address only air quality in the troposphere (lower
atmosphere). For treatment of issues related to stratospheric ozone depletion and global
climate change, see NRC (1998a, 2001a,b, 2003a) and NAE/NRC (2003).

Copyright © National Academy of Sciences. All rights reserved.

Air Quality Management in the United States 25
http://www.nap.edu/catalog/10728.html

INTRODUCTION

Emissions

Sources, present both outdoors and indoors, inclu de:
Vehicles, industry, utilities, consumer products, unpaved roads,
wildfires, and vegetation

Emission rates vary over time and space; they can be especi ally high
during of-f normal operating conditions (e.g., industrial process
malfunction) or unusual natural events

Physical Processing Chemical Processing

Dilution of pollutants into the local Chemical reactions in the atmosphere cause:
atmosphere • Destruction of some pollutants (e.g., CO,

Transport of pollutants up to 1000s some HAPs)
of kilometers • Formation of secondary pollutants

Atmospheric deposition to the Earth’s (e.g., O3, some PM components)
surfaces including soils, plants, and • Cycling from one chemical form to another
bodies of water
Some species react rapidly (a few minutes),
others remain in the air for many months

Pollutant Mix in the Atmosphere

Regulated pollutants, including:

Criteria pollutants

• Sulfur dioxide (SO2) • Particulate Matter (PM10 and PM2.5)
• Nitrogen dioxide (NO2) • Carbon Monoxide (CO)
• O3 and its precursors NOx and VOCs
• Lead (Pb)

188 Hazardous Air Pollutants ( HAPs)
Substances that contribute to acid rain including SO 2 and NOx
Ozone-depleting species, including cloroflurocarbons (CFCs)

Other species not currently regulated (e.g., methane, ammonia, other potential HAPs)

Human Exposure Ecosystem Exposure

Humans are exposed to air pollution Atmospheric deposition of air pollutants
• Outdoors (e.g., O3, acidifying compounds, and
• Indoors and in vehicles mercu3ry) to soil, plant, and water
• Routes other than air surfaces

Health effects are caused by acute (short - Multimedia transfer of pollutants through
term) and chronic (long-term) exposures water, soil, and food chains

Visibility Health Ecosystem Materials
Impairment Effects Effects Degradation

(e.g., monuments, buildings)

FIGURE 1-1 Schematic of the factors influencing the pollutant mix in the atmo-
sphere and the resultant impacts of pollution. Greenhouse gases and climate change
impacts are not included because they fall outside the committee’s charge.

Air pollutants are often characterized by how they originate: pollutants
emitted directly into the atmosphere are called primary pollutants; those
formed as a result of chemical reactions within the atmosphere are called
secondary pollutants. Control of secondary pollutants is generally more
problematic than that of primary pollutants, because mitigation of second-

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26 AIR QUALITY MANAGEMENT IN THE UNITED STATES

ary pollutants requires identification of the precursor compounds and their
sources as well as an understanding of the specific chemical reactions that
result in the formation of the secondary pollutants. Control can be further
complicated when the chemical reactions resulting in secondary-pollutant
formation involve complex, nonlinear interactions among the precursors.
Under those conditions, a 1:1 relationship might not exist between a reduc-
tion in precursor emissions and reductions in secondary-pollutant concen-
trations. Ground-level O3 is an example of such a secondary pollutant; it is
formed by reactions of nitrogen oxides (NOx) and volatile organic com-
pound (VOC) species3 in the presence of sunlight. In some circumstances,
O3 concentrations are most effectively controlled by lowering both VOC
and NOx emissions. For other circumstances, lowering VOC or NOx emis-
sions may be most effective (NRC 1991).

Similar complications arise in the mitigation of suspended particulate
matter (PM), which refers to a heterogeneous collection of solid and liquid
particles that include ultrafine particles (diameters of less than 0.1 mi-
crometers [µm]); fine particles (diameters of 0.1 to a few micrometers),
which are commonly dominated by sulfate, nitrate, organic, and metal
components; and relatively coarse particles (diameters of a few micrometers
or more), which are often dominated by dust and sea salt. PM can be a
primary or secondary pollutant. As a primary pollutant, PM is emitted
directly to the atmosphere, for instance, as a result of fossil fuel combus-
tion. As a secondary pollutant, PM is formed in the atmosphere as a result
of such processes as oxidation of sulfur dioxide (SO2) gas to form sulfate
particles. Because the reactions that result in the formation of secondary
PM often depend on the concentration and composition of preexisting
airborne PM, control strategies that lower the emissions of one chemical
constituent of airborne PM might not affect or might in some cases increase
the concentrations of other components of PM. Even though pollutants
have been typically treated independently in many of the air quality regula-
tions in the United States, pollutants are often closely coupled. For ex-
ample, most pollutants are emitted into the atmosphere by the same source
types (see Figure 1-2). They also often share similar precursors and similar
chemical interactions once in the atmosphere. For example, many of the
VOCs that react to form O3 are also identified as hazardous air pollutants

3Nitric oxide (NO) and nitrogen dioxide (NO2) are referred to together as NOx. VOCs are
organic compounds present in the gas phase at ambient conditions. Several other terms are
used operationally to refer to and classify organic compounds. For example, reactive VOCs
are sometimes designated as reactive organic gases (ROG); however, because hydrocarbons
make up most of the organic gas emissions, this category is also called reactive hydrocarbons
(RHC). Moreover, because methane dominates the unreactive category, the term nonmethane
hydrocarbons (or NMHC) is often used. Unless noted otherwise, VOCs will be used in this
report to represent the general class of gaseous organic compounds.

Copyright © National Academy of Sciences. All rights reserved.


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